Figure 1. Location of the adenosine deaminase (ADA) binding site for CD26/dipeptidyl peptidase IV.

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Human and bovine ADA bind CD26, but mouse ADA does not. Catalytically active human-mouse ADA hybrids were constructed, expressed, and used to localize residues necessary for CD26 binding to the a2 helix (aa 126-143) (Richard et al J Exp Med 192:1223, 2000). The presence of Arg142 in human ADA and Gln142 in mouse ADA largely determine the ability or inability, respectively, to bind CD26. The RasMol model is based on coordinates for murine ADA crystal structure (Wilson et al Science 252:1278, 1991). The a2 helix (aa 126-143) is shown in orange, with space-filling display of murine residues D131, Q142, and A143. For orientation the C-terminal helix (residues 337-351) is shown in blue. Bound inhibitor (rose) and the zinc ion (yellow) are shown as space-filling structures at the active site. The panel compares partial sequences (aa 115-150) of human, murine, and bovine ADA. Amino acids 126-143 of human ADA, green letters; D131, Q142, and A143 of murine ADA, red letters; other aa of bovine and murine ADA that differ from the human sequence, blue letters. (Adapted from Fig 3 of Richard et al J Exp Med 192:1223, 2000).